I actually say "good luck" pretty generally (you can never have too much luck). If you're referring to specific action they're about to undertake and you want to emphasize hard work, "Make it so" is another situationally useful phrase.
Slightly more generally useful might be "Make the future happen."
Is your goal to increase the emphasis on hard-work and rationality, or just have something that sounds nice that you don't disagree with? (In that case I'd just go with "good luck").
Is there any rationalist equivalent of "good luck"? I've tried a few variants, such as "work well", "knock them dead", "we're with you" and certain situation-specific phrasings, but haven't found anything that worked generally - though a hearty "may all the gods of Olympus be with you!" can serve. Not a vitally important point, but it would be nice to have something similarly supportive and yet accurate to say.